1. Field of the Invention
This invention generally relates to monitoring and controlling the state of a thermal process and in particular to the real time detection and control of a thermal state of a liquid in a vessel at or below its boiling point on a gas cooktop burner by applying a modulated heat input and measuring and processing the thermal dynamic response that results.
2. Description of Related Art
Many processes for industrial, commercial, and residential applications involve the supplying of energy into an often inhomogeneous media. In some examples such as cooking, such a process might even give rise to chemical changes. The process involves radiation, convection, and phase transformation, and often is chaotic. Thus, to perform a simple temperature control is untenable. Even the most common situations such as simple boiling of water are chaotic and therefore extremely complicated. In the past, numerous attempts to keep track of the cooking process by means of various sensors have not been successful. A common user requirement is to maintain a steady simmer or boiling condition in a liquid. A controller which provides only temperature regulation cannot reliably meet this requirement because its setpoint temperature must be set to closely match the boiling point of the particular liquid, e.g. it must account for a temperature drop across the vessel wall, amount of salt present and altitude above sea level. If the commanded temperature is not accurately set, a temperature regulator will either fail to bring about the desired simmer/boil state or will continuously overheat the subject by trying to reach a temperature above the boiling point of the liquid contents.
Two prior art attempts to construct a smart cooktop have met with only limited success. The first attempt used a capillary-type thermostatic valve with a spring loaded sensor at the center of the burner to control temperature of the pot surface; it is described in a field service publication for a Model FA-C combination gas-cock and thermostat published in 1965 by Robertshaw-Controls Co., of Youngwood, Pa. USA which was used in a gas range Model 73YXT manufactured by Caloric Corporation of Topton, Pa. USA. Some versions incorporated an adjustment to control maximum flame size. This system was fairly good at temperature control; however, it was not sufficiently accurate to hold a low boil nor could it detect changes in the boiling point or a boil dry condition. The second attempt described in a 1995 brochure "Simmer Sentry" of the Gas Research Institute of Chicago, Illinois USA, was more sophisticated, incorporating both acoustic and temperature sensors in a "wand" that was plugged into the range control panel and immersed in the pot. The intrusive nature of the "wand" was inconvenient, and the performance was less than desired.
In U.S. Pat. No. 4,646,913, issued to Romeo Delotto et al., on Mar. 3, 1987, and assigned to Indesit Industria Electtrodomestici Italiana S.P.A., a sensor group is described for automatic temperature control for foods in vessels heated by a flame from a gas burner that contacts the vessels' bottom. The burner includes a vertical nozzle and the nozzle axis and sensing group are both off center with respect to the burner axis. Foods in the vessels are progressively heated and their temperature is sensed by a first sensor which when boiling occurs remains constant. A second sensor, which controls a gas supply line, senses increasing temperature slightly but continuously and limits the gas supply. Because this approach is dependent upon progressive heating of the vessel's contents, it would not be suitable where substances are added to the vessel contents or where stirring of the vessel contents replaces hot fluid with cold fluid near the sensor.